1. Field
The present invention relates to a printed circuit board (PCB) to which a flexible cable including transmission lines having different characteristics is connected at a number of points by using an anisotropic conductive film (ACF).
2. Description of the Related Art
In general, electronic equipment incorporates a printed circuit board (PCB) formed of rigid resin, on which various components are mounted, and circuit lines electrically connecting the components are formed. Also, a notebook computer incorporates various circuit boards including a PCB such as a motherboard. Those circuit boards are electrically connected to each other by a flexible circuit board (which will be hereinafter referred to an FPC wherein circuit lines are provided in a resin sheet), etc.
Furthermore, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-162120, one end of each of two rectangular FPCs is fixed to a rectangular PCB having upper and lower surfaces on which circuit line patterns are formed, respectively, such that the longitudinal directions of the rectangular FPCs are perpendicular to each other. In each of the upper and lower surfaces of each of the FPCs, a number of circuit patterns are formed to extend in the longitudinal direction of each FPC. On the other end sides of the rectangular FPCs, parts of circuit patterns which electrically connect the FPCs are exposed. After the FPCs are bent, the other end sides of the FPCs are made to overlap each other, and then they are adhered to each other by an anisotropic conductive adhesive, so that the exposed parts of the circuit patterns are electrically connected to each other.
According to Jpn. Pat. Appln. KOKAI Publication No. 7-162120, predetermined circuits of the circuit patterns formed on the upper and lower surface sides of the PCB can be electrically connected to each other without provision of through holes or jumper wires.
In recent years, the speed of information processing or operation speed has been increased. As a result, there is a case where in a single PCB, circuit lines having different impedances are formed for a single signal line, which can be shared by a plurality of signals since the timings of transmission of the signals are made to differ from each other. In particular, it should be noted that specific high-speed transmission lines are provided, which differ from ordinary transmission lines for use in information processing, which comply with IEEE 1394 or USB 2.0.
Further, if, e.g., a graphic signal, is transmitted through such a high-speed transmission line as stated above, it is necessary to form at least 16 lanes, i.e., 64 signal lines. Furthermore, in a USB, since a plurality of connectors are provided, circuit lines extending to the connectors are formed. In a PCB, since various components are mounted thereon, circuit lines tend to be located to extend between the components or be located on an outer peripheral area of a surface of the PCB. Therefore, the larger the number of high-speed transmission lines provided on the PCB to improve the function of the PCB, the larger the area for provision of the high-speed transmission lines, thus increasing the size of the PCB. In order to avoid such a problem, a PCB or an FPC is formed to have a multi-layer wiring structure. However, there is a case where the multi-layer wiring structure is limited with respect to the number of times a signal is transmitted through a through hole or holes when being transmitted between layers. Whether it is limited or not depends on a signal to be transmitted. For example, in a PCI Express bus, it is preferable that the number of times a signal is transmitted through the through hole or holes be only one.
A technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-162120 includes a manufacturing step in which in space, ends of FPCs are positioned relative to each other, and are then fixed to each other. Thus, this manufacturing step cannot be easily carried out. To be more specific, in the technique disclosed in the KOKAI Publication, after two EPCs are prepared, and ends of the FPCs are fixed to a PCB such that they are made to overlap each other to intersect each other at right angles, the other ends of the FPCs are adhered to each other in such a way as to intersect each other at right angles. That is, in a step, the one ends of the FPCs must be accurately fixed to the PCB, so that in another step, the other ends of the FPCs can be made to intersect each other at right angles. Even if one of the FPCs is slightly inclined, they cannot be located to intersect each other at right angles, and contacts cannot be located to face each other when the other ends of the FPCs are put together, that is, they cannot be electrically connected to each other. However, the above other ends of the FPCs can be forcibly adhered to each other, while being made to intersect each other at right angles, since the FPCs have flexibility. However, in this case, since they are adhered against their reactive forces, there is a risk that the adhered parts may be separated by secular changes, etc., and as a result electrical disconnection may occur.
Moreover, in Jpn. Pat. Appln. KOKAI Publication No. 7-162120, FPCs are required to be positioned with a higher precision as the number of circuit lines is increased, and the pitch of the circuit lines is decreased. However, the FPCs cannot be easily connected to each other in space with a high precision. In addition, widths of circuit lines serving as high-speed transmission lines are determined based on their impedances, etc. That is, they cannot be freely increased, even if they need to be done in order that the above positioning be easily performed.